Paladins are a class from Dungeons and Dragons. They are divine warriors of a somewhat more martial bent than Clerics, receiving a variety of powers generally focused around smiting the enemies of their god, and tend to have high Charisma scores to fuel their holy powers. Many a fictitious maiden has benefited from a Paladin's Laying On of Hands. (Purely to heal their injuries, you understand. Unless they're into that sort of thing, of course.)

The burden Paladins bear (following a strict code of conduct or being knocked back to a Fighter without bonus feats) is often abused by dickish DMs, who seem to derive some sort of twisted pleasure out of forcing a player's Paladin into hideously arbitrary situations where any choice they take results in a fall from grace and the stripping of powers. Even otherwise reasonable DMs seem to turn into complete dicks when presented with a chance to make a paladin player's life a misery, and getting an atonement spell can become a chore if he's feeling particularly unpleasant. As a result, many avoid playing paladins entirely due to bad experiences with evil DMs or terrified players who act like "party cops" out of the fear of the DM de-paladining them.

Your party will never have to deal with the dilemma of "you can only save one: the baby or the nun" situations unless you have a paladin in the group.

There is none of that, "Oh well if you're truly sorry, there's nothing I can do." horseshit. No, he coup de graces your ass because he's a goddamn paladin. His job is killing evil. You know what his job doesn't entail? Being a sympathetic ear for every whiny NE or CN or LE douchebag who's only being evil because the world is unfair to him or every punk that lets his own dislikes or laziness overcome his own personality. You know what unfair is? Being able to know what kind of person everyone is before you even talk to them. Smelling evil so potent on a motherfucker that you want to sink your fingers in his chest and pull that tar out until the screaming stops. Having the psychotic urge to murder people that you've never even met, for the sole reason that your God decided that you ought to be his right hand without your choice in the matter, that's unfair.

But unlike Evil McBlacknails over there, that Paladin puts on his helmet, sharpens his sword, and then continues walking through crowds of people day by day, resisting the urge. Seeing evidence of injustice so black it makes him sick. Seeing murderers and rapists walk the street, watching good men hang as evil ones pull the lever. Saving his righteous violence for when the situation exactly, specifically, precisely calls for it. Surgically removing that which is most evil. Because he's a Paladin. And if he gave in to the urge, what would he be? Who will right the true wrongs if not he? It's not about not falling as a Paladin. It's about falling so fucking hard you crash through the planet and stand up on the other side.' - Dragonladieshere

Some of the misconceptions that I am aware of some people having are: self-righteously throws fights by using the word "dirty" to refer to "realistic fighting," doesn't care about murderous tyrants as long as they gave themselves legal permission, believes that strategic retreats are always "cowardly," doesn't believe in letting the other people fight when "talking" would be more "right" in a "fighting" game."Counter: A paladin’s code of honor is not about throwing fights; it’s about not starting them. If someone is as evil and dangerous as you think they are, then will have no problem throwing the first blow, and if they do not do so, then perhaps they are not as dangerous as you think. How many have been killed in fights that they picked with somebody whom they FALSELY believed would’ve attacked them first, but who in fact had no intention of doing so until he himself was attacked and had to defend himself? And yet, how many people have killed in self-defense in the same circumstance, when they in fact could’ve simply incapacitated their attacker and learned that his only real crime was stupidity?More importantly, a paladin learns to pick her battles, but BEFORE the battle actually starts. If you attack something that you know to be dangerous, and then run off without planning to finish the job, then you have put others in danger by angering the enemy you attacked and encouraging him to lash out. If you plan to help people by deposing a tyrant, and you don’t bother gathering enough allies to ensure that you actually defeat him when you engage him, then the tyrant needs to know that when – not if – he defeats you, his quarrel with you will be finished, and he needs not burn entire villages to the ground looking for where you fled to and who helped you. If he was not the kind of ruler who would do that after a half-assed assassination attempt, then you would not have needed to depose him in the first place, and thus, if you are stupid enough not to bring enough allies to ABSOLUTELY guarantee victory, then you would need him to know that you acted alone and never had a chance worth him getting worried about after you are dead.' - Beldak_Serpenthelm

Lawful Good. If you fuck up at being Lawful Good, you're busted down to Fighter at the same level, and your church shuns you. The controversial Unearthed Arcana book adds the chance to play Paladin-Cavaliers, who are incredibly ridiculous in power level (can stay conscious and retreat at negative hit points, can boost Str, Con, Dex, and Cha a little bit each level up, immunity to fear, etc).

A sub-class of Warrior, 2e Paladins are notable primarily by their potential to use Holy Avenger swords, which inflict an additional +10 damage versus Chaotic Evil foes (which is a lot for 2e), and create a circle of power that is a selective antimagic field versus lower level magical effects (so all enemy buffs and enemy magic items created by a level 12 wizard power down when a level 13 paladin walks up). The Cavalier kit recreates about half of the abilities they used to have for a Paladin; its notable that the Cavalier is where the fear immunity for a 3e Paladin comes from, not the Paladin. A Paladin probably does not have a good chance of being worthwhile compared to a fighter if they cannot expect to find their holy sword, however.

Lawful Good, with an explicit Code of Conduct, in exchange gains a number of immunities to various status effects, their famous ability to Smite Evil, and the service of an intelligent and useful mount. Fuckup paladins who decide to embrace evil had the option to multiclass into the evil prestige class Blackguard (described in the DMG), which receives bonus abilities if the character trades in levels of Paladin, and mostly gains the same power but backwards. (Immunity to the symptoms of diseases so they can act like a Typhoid Mary, Smite Good, etc.) Some settings strip the alignment restrictions off, notably Eberron or provide non-neutral alignment-based alternatives, such as Unearthed Arcana. These are: the Paladin of Freedom (Chaotic Good), the Paladin of Slaughter (Chaotic Evil, cornflakes flavored), and the Paladin of Tyranny (Lawful Evil). There are also options to make the Paladin more cleric-like in Dungeonscape, or completely remove spells such as in Complete Warrior.

Pathfinder made a number of mechanical improvements (beefy ones, Pathfinder paladins are badass), but mostly left them alone conceptually. Now they cast spells off charisma rather than wisdom (and praise Saerenrae for that), and their Smites, on top of granting better defensive and offensive bonuses, keep Smiting until they either Smite something else or the Smitee is dead. (Sure, it can theoretically run out in twenty-four hours, but almost nothing suffering from a Smite is going to live that long). The Code of Conduct was also softened a little to allow paladins to more easily be team players and not 100% stick-in-the-mud party cops. One problem, if you can call it that, is that the paladin is perhaps too badass: he's more specialized than most of the other martials, but if the DM keeps throwing evil outsiders or undead into the campaign as BBEGs a modestly-well-prepared paladin will be able tear through them like a holy-powered buzzsaw without a lot of effort, since he deals better "burst" damage than almost any of them and has great saves and immunity to shit like mummy rot, so he won't be afraid to charge in and rip and tear when another class would be adverse to taking risks around one. If the final boss keeps getting one-shotted by a well-timed Smite Evil litany of righteousness power attack/deadly aim combo, it may very well be time to invest in "moral complexity." (Read: making the BBEG a neutral alignment so the pally can't just disintegrate him.)

In 4th Edition, the paladin must be the same alignment as their deity; no more Lawful Stupid out of fear of falling. The slightest deviation from one's alignment no longer results in a DM bitchslap and losing class features; instead, you get vague threats that the other faithful of the paladin's religion will seek you out to administer chastisement for your failings. You would think that since this removes one of the oldest mechanical complaints about the class, fans would be happy, but "Paladins must be Lawful Good!" is such a sacred cow that people were bitching over its loss even as they bemoan the Lawful Stupid Paladins and Orc Baby Dilemmas of old editions.

Paladin abilities are more focused on being a meat-shield than being a holy avenger; for more smite-evil action, you want the Avenger class from Player's Handbook 2. The paladin can also officially no longer fall. What this means is that there is no mechanical penalty to not adhering to your DM's definition of Lawful Good, the biggest reason why most paladins got played as Lawful Stupid in the first place, but of course, /tg/ likes to joke that this applies literally, making a meme out of 4e-paladins taking no falling damage.

In 5th Edition, paladins no longer must adhere to any alignment. However, when they reach 3rd level they swear their Paladin Oath, which gives him a code of conduct for him to follow. Also, since paladins have their codes of conduct clearly stated in the PH, rather than leaving it up to the nebulous personal decisions of a DM as to what actually constitutes "Lawful Good" and its required behavior, it's a lot harder for DMs to force a paladin to fall on grey area moralities.

There are three oaths: the Oath of Devotion, which makes him a stereotypical paladin "defend the weak, enforce justice, be honest" and gives him typical paladin spells; the Oath of the Ancients which makes him a more relaxed and nature-oriented paladin (who can turn into a treant at 18th level and stops aging at 14th); and the Oath of Vengeance, which turns him into Batman (in the whole "I hunt evil relentlessly" regard), gives him skills oriented towards making sure whoever he's fighting cannot leave ever, and allows him to turn into an actual angel of vengeance at 18th level.

Mechanically speaking, 5th ed paladins have a few minor differences; their passive "detect evil" is now 1+Cha mod uses per day, but retains its potency, meaning that the paladin can now be caught by surprise by evil if he doesn't think to check. Smiting now uses spell slots, with higher spell slots dealing more damage (which is kind of silly, because there are dedicated smiting spells).

There is also an Oathbreaker option listed, since paladins can't technically fall anymore, that details what happens to a paladin who has broken his sworn oath; this replaces the old Blackguard, with all kinds of nasty powers relating to demons and the undead.